It is well known that addition of slight amounts of Pb, Bi, or In can improve the electric characteristics of aluminum foil for electrolytic capacitors (See for example, Japanese patent publication No. 44-29577, Japanese patent Kokai No. 53-114059, and Japanese patent Kokai No. 57-110646).
Components such as Pb, Bi, and In enhance the pit generation of a capacitor foil, which then increases the capacitance of a capacitor foil. On the other hand, etching tends to create excessive pits on aluminum foil which reduces the strength of the foil. To resolve the problem, an aluminum foil was proposed by Japanese patent publication No. 62-42370, which contains Pb, Bi, and In in the surface layer but does not actually contain Pb, Bi, and In within the inner part of the foil.
The aluminum foil for capacitors described in Japanese patent publication No. 62-42370 consists of a surface layer having a depth of 0.1 .mu.m below the surface and containing at least one element selected from the group consisting of Pb, Bi, and In at a total concentration ranging from 0.005 to 0.2 wt %, while the inner part other than the surface layer contains no Pb, Bi, and In, or less than 0.0001 wt % for each element, and more preferably less than 0.00001 wt %. This layer composition induces the formation of a number of pits during the initial stage of etching, and let these pits continue to develop into the inner part consisting of high purity aluminum, and enhances the expansion of the surface area.
The presence of Pb, Bi, and In only in the surface layer enhances pit formation during the initial stage of etching. However, this compositional structure has the disadvantage that further progress of etching is inhibited preventing the development of pits beyond the thickness of the surface layer, thus making it difficult to obtain an enlarged surface area.